1. Field of the Invention
The present invention relates to microwave systems. More specifically, the present invention relates to methods and apparatus for waveguide switches utilized in high power microwave systems.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
In spacecraft and other applications, the reliability of certain systems is critical. Accordingly, many systems are designed with redundant components to ensure continued operation of the system. Waveguide switches are often used in communication systems to switch in a redundant component, circuit or system. Hence, the reliability of the switch is important as is the weight for spacecraft applications.
The reliability of waveguide switches for use with high power when constructed in accordance with conventional teachings is limited. One reason for the limited reliability is that prior waveguide switches have a small clearance gap between the rotor and the housing which measures under 0.002 inches (i.e., less than 2 mils). The clearance gap allows the rotor to turn within the housing on bearings without binding or frictional drag. Also, the gap produces a low impedance transmission line that is part of the low loss RF choke of the waveguide switch. The small gap, however, is prone to voltage breakdown and thus limits the power handling capability. Larger gaps result in degraded electrical performance of the switch rendering it unacceptable.
Another problem associated with waveguide switches of conventional design is heat generation. Some of the RF power passing through the switch is absorbed which generates heat in the rotor since the rotor is essentially an extension of the waveguide. Generally, in space applications, the rotor is cooled primarily by the radiation of the heat to the switch body. The rotor can be comprised of, for example, aluminum, magnesium or zinc. When the rotor is heated, it expands resulting in a net reduction of the size of the clearance gap. To ensure that the switch will operate when the switch is hot, the clearance gap must initially be made larger when at ambient temperature. Then the gap will be at the proper clearance when the switch is normally operating after RF power is applied.
A third problem associated with known waveguide switches operating in a vacuum is that at high power levels and in the presence of the correct combination of voltage, frequency and gap size, the multipaction phenomena can occur within the clearance gap. Similarly, during the propagation of electromagnetic waves in air, ionization of the gases in the air can occur in the presence of a high voltage (e.g., 72KV/inch) within the waveguide. The ionization of the gases can result in electrical break down, arcing and short circuiting between the waveguide walls. This phenomena can occur in waveguide switches for communication satellites which also operate in a vacuum.
Whereas at ambient pressure, the electric field across the small clearance gap can cause ionization breakdown estimated at approximately three kilowatts peak. As presently configured, the waveguide switch of the prior art is estimated to handle less than five-hundred watts average and peak power in vacuum. By comparison, WR75 waveguide can accommodate seven kilowatts peak in vacuum and in excess of four-hundred kilowatts peak power in air before breakdown occurs. Therefore, the presence of the clearance gap clearly limits the power handling capability of prior art waveguide switches. Furthermore, machining tolerances cause the clearance gap size in identical switches to vary. This, in turn, results in the insertion loss and the isolation of RF paths in identical switches to vary resulting in inconsistent switch parameters. It is noted that low loss and high isolation of RF paths are imperative characteristics for the successful operation of high power waveguide switches.
A fourth problem associated with known waveguide switches is the switch power rating. The waveguide switches of the past are limited in power switching capability which is approximately one kilowatt peak. If the waveguide switch is to be utilized for other than redundancy switching, the switch rating could be inadequate. For example, switching individual solid state power amplifiers in a channel requires a power switch rating of approximately 50 watts. Thus, conventional switches typically having a power switch rating of approximately 240 watts are adequate. However, use of prior art switches, for example, downstream of the output multiplexers of a communication repeater to switch many sets of channels associated with the output multiplexers, would certainly exceed the power switching capability of known waveguide switches.
Thus, there is a need in the art for an improvement in conventional waveguide switches for high power microwave systems.